The construction of immunogenic peptides or peptide conjugates is an active and ongoing research pursuit. The goals include production of reagent antibodies for research, for example in neurobiology, and production of synthetic vaccines for human or veterinary application. Small synthetic peptides are poor antigens and typically require covalent association with macromolecular carriers and administration with adjuvant in order to elicit an immune response. Carriers also provide T-cell epitopes necessary for cell-mediated response and for helper functions in the humoral response. When presented appropriately, synthetic peptides can elicit antibodies against large proteins which display the same peptide epitope within their sequence. Vaccine research further seeks to define those synthetic immunogens capable of inducing an antibody response that is also able to neutralize the infectious activity of a virus or other pathogen from which the protein is derived.
A significant effort has been devoted to discovery of general rules which govern the selection of protein epitopes by the immune system and development of methodology for mimicry of such epitopes with synthetic immunogens. Evidence has emerged suggesting that linear or discontinuous epitopes may be recognized and that these may adopt defined conformational states that are not readily duplicated in a synthetic peptide. Attempts to devise conformationally restricted peptides as superior antigens have also been given serious attention. In such approaches the structural context of a peptide sequence within a protein antigen is considered in producing a suitable mimic. Typically, the epitope may adopt a secondary structure such as a β-turn or a-helix. A similar structure may be induced in a small peptide by intramolecular covalent modification between two residues that constrains its conformational freedom. These ordered structures can be important as B-cell determinants. However, they are insufficient immunogens in the absence of helper T-cell determinants. Recently developed peptide vaccine models have incorporated T-cell epitopes in association with the B-cell epitope. Designs include simple tandem linear synthesis of peptides as well as increased epitope valency through coupling T-cell and B-cell peptides to a branched polylysine oligomer. The latter assemblies, referred to as multiple antigenic peptides, have shown promise as vaccines against various pathogens.
Unlike the conformationally defined B-cell epitopes, sequences recognized by T-cells undergo extensive processing to short linear peptide fragments before they are bound to a major histocompatibility complex (MHC) for recognition at the surface of an antigen-presenting cell. This elaborate processing mechanism depends on intracellular proteolytic activity and translocation of the products to the cell membrane. Synthetic peptide immunogens may not effectively participate in this process, despite the presence of the T-cell epitope. The immunogenicity of the molecule can be expected to correlate with the efficiency of natural processing of the T-cell epitope. Studies with linear synthetic peptides indicated that chimeric peptides containing T-cell and B-cell epitopes were superior immunogens when the B-cell epitope was amino-terminal. However, the reverse orientation has also been reported to produce a stronger immune response. A general rule may not be obvious since natural antigen processing probably accepts various orientations, including internal epitopes that require multiple processing steps to release the peptide. Also the efficiency of a construct may depend on many other factors, such as molecular context and flanking sequences that affect processing or presentation and the overall nature of the immunogen which can affect the functional pairing between several available T-cell and B-cell epitopes.
Further limitations to the use of synthetic peptides as vaccines result from the genetic restriction to T-cell helper function. Multiple MHC class II molecules encoded within the genome of a species are subject to allelic exclusion. A specific T-cell epitope may interact with only one or a few alleles of the MHC. Therefore individuals may respond differently to the immunogen despite inclusion of T-cell helper epitopes. Vaccine development must overcome the MHC-restricted response to provide the broadest possible response in an outbred population. In the murine model the T-helper cell responses are MHC-restricted and major haplotypes H-2b, H-2k, and H-2d are represented in several inbred strains. Certain T-cell epitopes are known to be recognized in the context of multiple MHC class II alleles and can thereby provide “promiscuous” T-helper stimulation. A number of epitopes, such as from tetanus toxin, measles virus, and Mycobacterium tuberculosis have been reported to be universally immunogenic. These may have significant benefit for subunit vaccine design.
As an alternative to chemical synthesis, molecular biological techniques can provide significant advantages for production of polypeptides that display both B-cell and T-cell epitopes. Expression of proteins from cloned genes obviates burdensome peptide synthesis, purification and conjugation chemistry typically used in production of immunogenic materials. Furthermore, the stochastic chemistry for preparation of peptide-carrier conjugates is replaced by the defined chemical structure provided by the genetic fusion. Therefore the epitopes can be introduced in ordered structures that have optimal and reproducible immunogenic properties. Considerations that arise in the development of optimal designs can be addressed at the genetic level. Thus, definition of the target epitopes and their flanking sequences, relative orientation and conformations of these sequences within the larger polypeptide, and epitope copy frequency can be established in the gene design.
Several recombinant host proteins have been successfully utilized for immune presentation of peptide epitopes. The E. coli maltose-binding protein (MalE) has been used to study the influence of location and orientation of inserted T-cell epitopes. The major coat protein (pVIII) of filamentous bacteriophage fd has been used for display of HIV B-cell epitopes at the N-terminus. The recombinant phage particles evoked a strong antibody response in mice, which cross-reacted with HIV strains and which is also capable of neutralizing the virus. These approaches promise to enhance the potential of subunit or synthetic vaccine models.